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  • 1
    Electronic Resource
    Electronic Resource
    Dehydrogenation and aromatization of methane in the absence of oxygen on Mo/HZSM-5 catalysts before and after NH4OH extraction (1996)
    Springer
    Catalysis letters 40 (1996), S. 207-214 
    Details
    ISSN: 1572-879X
    Keywords: Mo/HZSM-5 ; Mo species ; MoO3 crystallites ; Al2(MoO4)3 crystallites ; methane activation ; NH4OH extraction
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Mo/HZSM-5 catalysts show good catalytic reactivity in the absence of oxygen for the dehydrogenation and aromatization of methane at 973 K. The active Mo species were investigated by combining catalytic studies on Mo/HZSM-5 catalysts before and after NH4OH extraction with XRD, BET, NH3-TPD and TPR analysis. The XRD patterns show that Mo species are well dispersed on the zeolite surface. The specific surface areas decrease with increasing Mo loading but they can be restored to a large extent by NH4OH extraction. NH3-TPD results suggest that the Mo species prefer to deposit on the strong acid sites of HZSM-5 zeolite. TPR profiles show that there is a kind of Mo species which is easily reduced. No TPR peaks could be obviously observed if the Mo/ HZSM-5 catalysts were extracted by NH4OH solution. The results of NH4OH extraction experiment and other relevant characterization studies suggest that there are several kinds of Mo species deposited on the surface. By referring to the Mo species on Al2O3 supported MoO3 samples, we propose that the dissolvable Mo species in NH4OH solution are MoO3 crystallites and their aggregates in octahedral coordination, while the unsoluble Mo species mainly are Al2(MoO4)3 and MoO 4 2− in tetrahedrally coordinated form. The catalytic performance of Mo/HZSM-5 catalysts before and after NH4OH extraction illustrates that Mo species in small MoO3 crystallites with octahedral coordination form are active for methane activation in the absence of oxygen on Mo/HZSM-5 catalysts, while Mo species in tetrahedrally coordinated form is less active for the reaction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00815284
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  • 2
    Electronic Resource
    Electronic Resource
    Methane and ethane activation without adding oxygen: promotional effect of W in Mo-W/HZSM-5 (1996)
    Springer
    Catalysis letters 38 (1996), S. 39-43 
    Details
    ISSN: 1572-879X
    Keywords: acid-assisted heterolytic splitting of methane ; Mo/HZSM-5 ; W/HZSM-5 ; Mo-W/HZSM-5 ; promotional effect of tungsten
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The conversions of methane and ethane over Mo/HZSM-5 and W/HZSM-5 catalysts are compared. A reaction model for hydrocarbon formation over Mo/HZSM-5 catalysts is proposed, which involves heterolytic splitting of methane and a molybdenum-carbene intermediate. Ethene is shown to be the initial product of methane conversion, and it undergoes further reaction to form aromatics in a solid acid environment. The promotional effect of addition of tungsten in the Mo-W/HZSM-5 catalyst in methane conversion reaction suggests the formation of Mo-W mixed oxide. The product selectivity patterns of Mo/HZSM-5 and W/HZSM-5 catalysts in ethane conversion reaction are consistent with a dual-path model involving dehydrogenation and cracking (or hydrogenolysis) of ethane. The rates of both these reactions over Mo/HZSM-5 are higher than over W/HZSM-5.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00806897
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  • 3
    Electronic Resource
    Electronic Resource
    Oxidative coupling of methane by carbon dioxide: a highly C2 selective La2O3/ZnO catalyst (1996)
    Springer
    Catalysis letters 42 (1996), S. 149-153 
    Details
    ISSN: 1572-879X
    Keywords: carbon dioxide as the oxidant ; oxidative coupling of methane ; La2O3/ZnO
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract In the oxidative coupling of methane by carbon dioxide, La2O3/ZnO catalysts were found to have high C2 selectivity and good stability. The coupling selectivity on La2O3/ZnO is about 90%, which is much higher than that on pure La2O3 or ZnO. The consumption ratio of carbon dioxide to methane is approximately one. X-ray diffraction analysis reveals that the structural forms of the oxides are unchanged during the reaction. The reaction mechanism for C2 formation is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00810680
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  • 4
    Electronic Resource
    Electronic Resource
    Regeneration behaviors of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for C2H6 dehydrogenation with CO2 to C2H4 (1999)
    Springer
    Catalysis letters 62 (1999), S. 185-189 
    Details
    ISSN: 1572-879X
    Keywords: C2H6 ; CO2 ; Fe/Si‐2 catalyst ; regeneration
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The catalytic performance of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for conversion of C2H6 with CO2 to C2H4 was examined in a continuous‐flow and fixed‐bed reactor. The results show that the Fe–Mn/Si‐2 catalyst exhibits much better reaction activity and selectivity to C2H4 than those of the Fe/Si‐2 catalyst. Furthermore, the coking–decoking behaviors of these catalysts were studied through TG. The catalytic performances of the catalysts after regeneration for conversion of C2H6 or dilute C2H6 in FCC off‐gas with CO2 to C2H4 were also examined. The results show that both activity and selectivity of the Fe–Mn/Si‐2 catalyst after regeneration reached the same level as those of the fresh catalyst, whereas it is difficult for the Fe/Si‐2 catalyst to refresh its reaction behavior after regeneration.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019015509871
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  • 5
    Electronic Resource
    Electronic Resource
    Interaction between ammonium heptamolybdate and NH4ZSM-5 zeolite: the location of Mo species and the acidity of Mo/HZSM-5 (1995)
    Springer
    Catalysis letters 35 (1995), S. 233-243 
    Details
    ISSN: 1572-879X
    Keywords: Mo/HZSM-5 ; NH4ZSM-5 ; zeolite ; BrØnsted acid sites ; methane activation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Mo/HZSM-5 catalysts show high reactivity and selectivity in the activation of methane without using oxidants. Mo/HZSM-5 catalysts with Mo loading ranging from 0 to 10% were prepared by impregnation with an aqueous solution of ammonium heptamolybdate (AHM). The samples were dried at 393 K, and then calcined at different temperatures for 4 h. The interaction between Mo species and NH4ZSM-5 zeolite was characterized by FT-IR spectroscopy, differential thermal analysis (DTA) and temperature programmed decomposition (TPDE) and NH3-TPD at different stages of catalyst preparation. The results showed that if Mo/HZSM-5 catalysts were calcined at a proper temperature, the Mo species will interact with acid sites (mainly with BrØnsted acid sites) and part of the Mo species will move into the channel. The Mo species in the form of small MoO3 crystallites residing on the external surface and/or in the channel, and interacting with BrØnsted acid sites may be responsible for the methane activation. Strong interaction between Mo species and the skeleton of HZSM-5 will occur if the catalyst is calcined at 973 K. This may lead to the formation of MoO 4 2− species, which is detrimental to methane activation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00807179
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  • 6
    Electronic Resource
    Electronic Resource
    Methane dehydro‐aromatization over Mo/MCM‐22 catalysts: a highly selective catalyst for the formation of benzene (2000)
    Springer
    Catalysis letters 70 (2000), S. 67-73 
    Details
    ISSN: 1572-879X
    Keywords: zeolite ; MCM‐22 ; Mo/MCM‐22 catalyst ; methane dehydro‐aromatization ; benzene
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract A molybdenum‐modified MCM‐22 catalyst has been used for methane dehydro‐aromatization. The catalytic performance on this Mo/MCM‐22 catalyst is featured by a higher yield of benzene and a lesser yield of naphthalene in comparison with that on a Mo/HZSM‐5 catalyst under the same experimental conditions. Methane conversion of 10.0% and benzene selectivity of 80% over a 6Mo/MCM‐22 catalyst at 973 K was obtained. Based on the effect of contact time, it is suggested that the reaction is severely inhibited by the products, probably due to their strong adsorption and slow desorption. The Mo/MCM‐22 catalysts were characterized by XRD, NH3‐TPD and TPSR techniques. XRD patterns of the Mo/MCM‐22 catalysts confirmed that Mo species are highly dispersed on/in the MCM‐22 zeolite if the Mo loading is less than 10%. NH3‐TPD experiment shows that the MCM‐22 zeolite contains strong and exchangeable Brønsted acid sites. TPSR of methane revealed that there is an induction period during which the Mo species are reduced by methane and transformed probably into Mo2C or Mo2OxCy. It is concluded that the nature of the methane dehydro‐aromatization reaction over the Mo/MCM‐22 catalysts is similar to that on the Mo/HZSM‐5 catalysts. The unique pore systems, the proper acid strength of the MCM‐22 zeolite and the Mo species are factors important for methane dehydro‐aromatization over Mo/MCM‐22 catalysts.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019079603279
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  • 7
    Electronic Resource
    Electronic Resource
    MAS NMR, ESR and TPD studies of Mo/HZSM‐5 catalysts: evidence for the migration of molybdenum species into the zeolitic channels (2000)
    Springer
    Catalysis letters 66 (2000), S. 155-160 
    Details
    ISSN: 1572-879X
    Keywords: methane aromatization ; bifunctional catalysis ; molybdenum ; magnetic resonance
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract NH3‐TPD, MAS NMR and ESR spectroscopies were employed to investigate Mo‐modified HZSM‐5 catalysts prepared by impregnation. It was found that the modification of Mo ions results in a pronounced decrease in the intensity of 1H MAS NMR resonance originating from Brønsted acid sites in the zeolites and a distinct splitting of Mo5+ ESR signals, which is attributed to the interaction of Mo with the Al atom of the zeolite framework. This presents distinct evidence that Mo ions migrate from the external surface of the zeolite into the lattice channels during the impregnation and subsequent treatment. The remaining Brønsted acid sites associated with the migrated Mo ions form the bifunctional catalytic centers that may be responsible for the outstanding catalytic performance in methane aromatization.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019099607029
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  • 8
    Electronic Resource
    Electronic Resource
    Methane dehydro‐aromatization over a Mo/phosphoric rare earth‐containing penta‐sil type zeolite in the absence of oxygen (2000)
    Springer
    Catalysis letters 66 (2000), S. 161-167 
    Details
    ISSN: 1572-879X
    Keywords: methane dehydro‐aromatization ; Mo/HZRP‐1 catalyst ; NH3‐TPD ; 27Al MAS NMR
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The dehydro‐aromatization of methane over a Mo‐modified penta‐sil type high‐silica zeolite containing phosphoric and rare earth oxide (abbreviated as Mo/HZRP‐1) was investigated. As a modification of HZSM‐5, HZRP‐1 is also a good support for the preparation of Mo‐based zeolite catalysts, and is active for methane dehydro‐aromatization. Mo/HZRP‐1 catalysts are more active at high Mo loadings compared with Mo/HZSM‐5 catalysts. 27Al MAS NMR spectra of Mo/HZRP‐1 reveal that there are two kinds of framework Al in HZRP‐1. It is suggested that only the tetrahedral coordinated Al atoms in the form of Al–O–Si species in the zeolite, in the proton forms, are responsible for the formation of aromatics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019020211572
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  • 9
    Electronic Resource
    Electronic Resource
    Study on the induction period of methane aromatization over Mo/HZSM-5: partial reduction of Mo species and formation of carbonaceous deposit (1999)
    Springer
    Catalysis letters 57 (1999), S. 95-102 
    Details
    ISSN: 1572-879X
    Keywords: methane aromatization ; induction period ; Mo species ; partial reduction ; carbonaceous deposit
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The induction period of dehydrogenation and aromatization of methane over Mo/HZSM-5 was studied by combining a pulse reaction method with TPSR, UV laser Raman, and 13C CPMAS NMR techniques. BET and XRD results showed that Mo species were well dispersed on/in the zeolite. TPSR in CH4 stream revealed that Mo species were reduced in at least two different stages before the formation of benzene. TPR results were in agreement with TPSR results. The two stages might be attributed to the reduction of two kinds of Mo6+ species to low valence Mo species. One was polymolybdate MoO3, and the other was crystalline MoO3. UV Raman spectra showed the existence of octahedrally coordinated polymolybdate species. XRD, however, did not detect any crystalline MoO3, possibly because they were too small to be detected with this technique. Pulse reaction results indicated that pre-reduction of the catalyst and formation of carbonaceous deposit could shorten the induction period. It is concluded that the formation of active sites during the induction period via partial reduction of Mo6+ species and formation of carbonaceous deposit on partially reduced Mo species is of significance for methane aromatization over Mo/HZSM-5.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1019087313679
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  • 10
    Electronic Resource
    Electronic Resource
    Dehydrogenation and aromatization of methane under non-oxidizing conditions (1993)
    Springer
    Catalysis letters 21 (1993), S. 35-41 
    Details
    ISSN: 1572-879X
    Keywords: Dehydrogenation and aromatization ; methane ; MoHZSM-5 ; ZnHZSM-5 ; ZSM-5 zeolites
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The dehydrogenation and aromatization of methane on modified ZSM-5 zeolite catalysts has been studied under non-oxidizing conditions with a fixed bed continuous-flow reactor and with a temperature programmed reactor. The results show that benzene is the only hydrocarbon product of the catalytic conversion of methane at high temperature (973 K). The catalytic activity of ZSM-5 is greatly improved by incorporating a metal cation (Mo or Zn). H2 and ethene have been directly detected in the products with a mass spectrometer during TPAR. A carbenium ion mechanism for the activation of methane is suggested.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00767368
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